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<HR ALIGN=CENTER>
<P>
<UL>
<UL>
<UL>
<LI>
<A HREF="#E68E37" >Defining the Characteristics of Data Objects</A>
<UL>
<LI>
<A HREF="#E69E128" >Jet Data Access and Remote Data Objects</A>
<LI>
<A HREF="#E69E129" >Instances of Data Objects</A>
<LI>
<A HREF="#E69E130" >Persistent Member Objects</A>
<LI>
<A HREF="#E69E131" >Recordset Objects Created from Virtual Tables</A>
<UL>
<LI>
<A HREF="#E70E43" >Dynamic Recordset Objects</A>
<LI>
<A HREF="#E70E44" >Static Recordset Objects</A></UL>
<LI>
<A HREF="#E69E132" >Consistency Issues with Recordset Objects</A></UL>
<LI>
<A HREF="#E68E38" >Understanding the Properties and Methods of the DAO DBEngine Object</A>
<LI>
<A HREF="#E68E39" >Defining the Workspace and Database Objects</A>
<UL>
<LI>
<A HREF="#E69E133" >Properties and Methods of the Workspace Object</A>
<LI>
<A HREF="#E69E134" >Properties of the Database Object</A>
<LI>
<A HREF="#E69E135" >Methods Applicable to the Database Object</A>
<LI>
<A HREF="#E69E136" >Connecting to an Existing Jet Database</A></UL>
<LI>
<A HREF="#E68E40" >Using the TableDefs Collection and TableDef Objects</A>
<UL>
<LI>
<A HREF="#E69E137" >The Attributes Property of TableDef Objects</A>
<LI>
<A HREF="#E69E138" >Understanding Flags and Intrinsic Symbolic Constants</A>
<LI>
<A HREF="#E69E139" >Mapping Database Member Objects with the CDaoTableDef Collection</A>
<LI>
<A HREF="#E69E140" >Mapping the Fields and Indexes Collections</A></UL>
<LI>
<A HREF="#E68E41" >Using the QueryDefs Collection and QueryDef Objects</A>
<LI>
<A HREF="#E68E42" >Creating Tables with C++ Code</A>
<LI>
<A HREF="#E68E43" >Creating and Using Recordset Objects</A>
<UL>
<LI>
<A HREF="#E69E141" >Properties of Recordset Objects</A>
<LI>
<A HREF="#E69E142" >Methods Applicable to Recordset Objects and Collections</A></UL>
<LI>
<A HREF="#E68E44" >Summary</A></UL></UL></UL>
<HR ALIGN=CENTER>
<A NAME="E66E17"></A>
<H1 ALIGN=CENTER>
<CENTER>
<FONT SIZE=6 COLOR="#FF0000"><B>- 6 -</B>
<BR><B>Understanding the Access Database Engine and DAO</B></FONT></CENTER></H1>
<BR>
<P>The Microsoft Jet Data Access Object (DAO) will become Visual C++ 4.0's primary method of connecting to and manipulating data that is stored in desktop and client-server databases. DAO is the term used by Microsoft and by this book to describe the container (the base or master class) for all the data-related objects discussed in this chapter. The first part of this book gave you a brief introduction to the DAO and its member objects. This chapter describes the structure of the Jet DAO in detail, because the member objects of the DAO constitute the foundation on which the majority of your Visual C++ database applications are built. This chapter features examples that use the DAO's member objects to create instances of objects with C++ code and display the properties of the objects in list boxes. By the time you complete this rather lengthy chapter, it's very likely that you will have learned more than you ever wanted to know about data-related objects—or, more simply, data objects.
<BR>
<BLOCKQUOTE>
<BLOCKQUOTE>
<HR ALIGN=CENTER>
<BR>
<NOTE><B>NOTE</B>
<BR>
<BR>There is not a one-to-one mapping between DAO objects and the MFC DAO classes. Wherever possible, I will show where a DAO object's functionality is found in an MFC DAO class member that is different.</NOTE>
<BR>
<HR ALIGN=CENTER>
</BLOCKQUOTE></BLOCKQUOTE>
<BR>
<A NAME="E68E37"></A>
<H3 ALIGN=CENTER>
<CENTER>
<FONT SIZE=5 COLOR="#FF0000"><B>Defining the Characteristics of Data Objects</B></FONT></CENTER></H3>
<BR>
<P>In the object-oriented terminology of OLE, objects are <I>containers</I> for properties, methods, and other objects. Object <I>properties</I> are member function pairs of a programmable object; you can set or return information about the state of a programmable object, such as the value of a data item in the field of a table. One member function sets the data, and another member function returns the data—thus the term <I>pair</I>. An object <I>method</I> is a single member function of an object. Methods perform an action on the object, such as changing the object's color, size, or shape. All member functions are said to be <I>encapsulated</I> in an object. You don't need to know the internal operations of the object to create an instance of the object and to manipulate the object in C++ code. All you need is a description of the properties and methods exposed by the object.
<BR>
<BLOCKQUOTE>
<BLOCKQUOTE>
<HR ALIGN=CENTER>
<BR>
<NOTE><B>NOTE</B>
<BR>
<BR>Technically, you should be able to alter any property of a programmable object by assigning an appropriate value to the Set member of the function pair. The ability to set property values under specific conditions depends on the type of object and the application in which the object is used. Access 1.x, for example, had many objects whose properties could be set only in design mode. Access 2.0 and Access 7.0 have far fewer of these "frozen" objects. Visual C++ 4.0 doesn't even have a design mode, as do Visual Basic and Access.</NOTE>
<BR>
<HR ALIGN=CENTER>
</BLOCKQUOTE></BLOCKQUOTE>
<P>The Jet DAO is an OLE Automation in-process server that provides an object-oriented wrapper for the DLLs that comprise the Jet database engine. OLE Automation provides indirect access to properties and methods of programmable objects through a set of predefined interfaces. As a Visual C++ 4 programmer, you don't have to take any special steps to use the DAO features. Figure 6.1 shows AppWizard creating a DAO database project called DataDict.
<BR>
<P><B><A HREF="06vcg01.gif" tppabs="http://202.113.16.101/%7eeb%7e/Database%20Developer's%20Guide%20with%20Visual%20C++%204,%20Second%20Edition/06vcg01.gif">Figure 6.1. Visual C++ 4.0's AppWizard creating the DataDict project.</A></B>
<BR>
<P>OLE Automation server applications are selective about which programmable objects and member functions are accessible to other applications. Making member functions of OLE Automation server applications accessible to OLE Automation container applications is called <I>exposing</I> the member function. OLE Automation servers have two classes of functions, Public and Private. Only Public functions are exposed to OLE Automation client applications, such as Visual C++. Once you create a reference to an OLE Automation server object, Visual C++'s Class Browser provides a convenient list of the collections and objects exposed by the server, plus the member functions of each object. Figure 6.2 shows the CDaoRecordset constructor in the Visual C++ 4 Browser. The syntax for the selected method or property appears to the right of the ? button, which opens the help topic for the property or method, as shown in Figure 6.3.
<BR>
<P><B><A HREF="06vcg02.gif" tppabs="http://202.113.16.101/%7eeb%7e/Database%20Developer's%20Guide%20with%20Visual%20C++%204,%20Second%20Edition/06vcg02.gif">Figure 6.2. Visual C++ 4.0's Browser displaying the syntax for the CDaoRecordset::CDaoRecordset() constructor.</A></B>
<BR>
<P><B><A HREF="06vcg03.gif" tppabs="http://202.113.16.101/%7eeb%7e/Database%20Developer's%20Guide%20with%20Visual%20C++%204,%20Second%20Edition/06vcg03.gif">Figure 6.3. The online help topic for the CDaoRecordset::CDaoRecordset() constructor.</A></B>
<BR>
<P>The Data Access Object classes in Visual C++ 4.0 are implemented by the seven main CDao... classes. (See Chapter 13, "Understanding MFC's DAO Classes," for more information on the MFC DAO classes.) These classes include all the objects that let you create, connect to, and manipulate the supported database types. This book uses the term <I>compound object</I> to describe an object that contains other objects to maintain consistency with OLE's compound document terminology. Like OLE's compound documents, compound objects have a hierarchical structure. Objects that are contained within other objects are called <I>member objects</I> of the container object. Visual C++ 4.0 treats member objects as properties of the container object.
<BR>
<P>Figure 6.4 illustrates the hierarchy of Visual C++ 4.0's DAO database classes. Access 2.0 and 7.0 have Forms, Reports, Scripts (macros), and Modules Documents collections that aren't supported in Visual C++ 4.0. In Visual C++ 4.0, Container objects and Documents collections are used to secure Jet databases in conjunction with System.mdw workgroup files that you create with Microsoft Access. (Visual C++ 4.0 can't create a workgroup file, previously called a system file.) Note that the CDaoFieldExchange class isn't derived from CObject.
<BR>
<P><B><A HREF="06vcg04.gif" tppabs="http://202.113.16.101/%7eeb%7e/Database%20Developer's%20Guide%20with%20Visual%20C++%204,%20Second%20Edition/06vcg04.gif">Figure 6.4. The hierarchy of Visual C++ 4.0's Jet 3.0 Data Access Object.</A></B>
<BR>
<P>The following sections describe the version of the Jet DAO included with Visual C++ 4.0, how you create member objects of the data access object, and one method of classifying these member objects (by their persistency). As is the case for many other disciplines, the taxonomy of database objects isn't a settled issue. Detailed information on the properties and methods of the data objects discussed in the following sections appears later in this chapter.
<BR>
<BLOCKQUOTE>
<BLOCKQUOTE>
<HR ALIGN=CENTER>
<BR>
<NOTE><B>CAUTION</B>
<BR>
<BR>You might find this chapter to be somewhat difficult. It has a grand mix of DAO terminology and MFC DAO terminology. For example, there is such a thing as a DAO Recordset, and there is also an MFC DAO CDaoRecordset class. Most of the methods and properties that are contained in the DAO Recordset are found as either member functions or member variables in the MFC DAO CDaoRecordset class object. However, that one-to-one relationship doesn't always carry through. Some of the native DAO methods and properties are simply not supported in the MFC DAO classes. I will tell you when a feature of DAO isn't directly supported.
<BR>
<BR>For unsupported features, you can call directly to the DAO engine. See Books Online, MFC Technical Notes, Number 54 for more information about interacting directly with DAO. It will infrequently be necessary to directly interact with DAO.</NOTE>
<BR>
<HR ALIGN=CENTER>
</BLOCKQUOTE></BLOCKQUOTE>
<BR>
<A NAME="E69E128"></A>
<H4 ALIGN=CENTER>
<CENTER>
<FONT SIZE=4 COLOR="#FF0000"><B>Jet Data Access and Remote Data Objects</B></FONT></CENTER></H4>
<BR>
<P>Visual Basic programmers will realize that there are actually three different versions of DAO. Visual C++ 4 is shipped only with DAO 3.0, a 32-bit version of DAO. MFC 4's DAO classes are only 32-bit. For programmers who are still developing 16-bit applications, DAO is just not available.
<BR>
<P>Here are the three versions of DAO (each of which can be found in your Windows 95 \Program Files\Common Files\Microsoft Shared\Dao folder):
<BR>
<UL>
<LI>The Microsoft DAO 3.0 Object Library is the standard 32-bit OLE Automation "wrapper" (Dao3032.dll) for the Jet 3.0 database engine (MSJT3032.DLL in \Windows\System). Dao3032.dll is included with Visual C++ 4.0 and can be used only with 32-bit applications. Microsoft Access 7.0 uses the DAO 3.0 Object Library and the Jet 3.0 database engine.
<BR>
<BR>
<LI>The Microsoft DAO 2.5/3.0 Compatibility Library is an alternative 32-bit OLE Automation type library (Dao2532.dll). Dao2532.dll isn't included with Visual C++ 4.0.
<BR>
<BR>
<LI>The Microsoft DAO 2.5 Object Library is a 16-bit OLE Automation wrapper (Dao2516.dll) for the Jet 2.0 database engine of Access 2.0 (Msajt200.dll). Dao2516.dll isn't included with Visual C++ 4.0 and is of no use to Visual C++ programmers because DAO isn't supported by the 16-bit version(s) of MFC.
<BR>
<BR>
</UL>
<P>Figure 6.5 shows the relationships between the Jet database engine used by Microsoft Access 1.1, 2.0, and 7.0 (Access 95), and Visual C++ 4.0.
<BR>
<P><B><A HREF="06vcg05.gif" tppabs="http://202.113.16.101/%7eeb%7e/Database%20Developer's%20Guide%20with%20Visual%20C++%204,%20Second%20Edition/06vcg05.gif">Figure 6.5. A simplified comparative diagram of the 16-bit and 32-bit implementations of the Jet database engine.</A></B>
<BR>
<P>With Visual C++ 4, the programmer is limited to using DAO 3.0. With DAO 3.0, you can create, open, or attach tables from 1.0, 1.1, 2.5, and 3.0 Jet databases. There's not too much in the way of limitations. Follow these guidelines:
<BR>
<UL>
<LI>Use the CompactDatabase() member function of the CDaoWorkspace class to convert Jet databases from one version to another for use with Visual C++ 4.0. The CompactDatabase() function doesn't convert Access-specific objects, such as forms, reports, macros, and modules, from one version to another.
<BR>
<BR>
<LI>Don't use the CompactDatabase() method to convert Jet 1.0, 1.1, or 2.5 .MDB files to version 3.0 if you plan to use the Jet 3.0 .MDB file with Access 7.0. Access 7.0 can't open an .MDB file that you convert from an earlier version to version 3.0 with Visual C++ 4.0's CompactDatabase() method.
<BR>
<BR>
<LI>Visual C++ 4.0 can't create workgroup (System.mdw) files that are necessary to secure .MDB databases. You need a copy of Access 7.0 to create a 32-bit workgroup file or a copy of Access 2.0 to create a 16-bit system file (SYSTEM.MDA). Workgroup or system files usually reside on a workgroup or file server, together with shared .MDB files.
<BR>
<BR>
<LI>Don't convert existing 16-bit Access system files (SYSTEM.MDA) to 32-bit workgroup files (System.mdw) until all users of the system file have converted to Jet 3.0 databases. You can attach a 16-bit SYSTEM.MDA file to a Jet 3.0 database without difficulty.
<BR>
<BR>
</UL>
<BLOCKQUOTE>
<BLOCKQUOTE>
<HR ALIGN=CENTER>
<BR>
<NOTE><B>NOTE</B>
<BR>
<BR>Use Access's database conversion feature to upgrade versions of .MDB files that contain Access-specific objects. Converting Access 1.0 and 1.1 files is a two-way process. However, once you use Access to convert a version 1.0 or 1.1 .MDB file to Access 2.0 format, or convert a 1.0, 1.1, or 2.0 file to Access 7.0 format, the process isn't reversible. It's a safer practice to use the appropriate version of Access for all Jet database conversion operations.</NOTE>
<BR>
<HR ALIGN=CENTER>
</BLOCKQUOTE></BLOCKQUOTE>
<BR>
<A NAME="E69E129"></A>
<H4 ALIGN=CENTER>
<CENTER>
<FONT SIZE=4 COLOR="#FF0000"><B>Instances of Data Objects</B></FONT></CENTER></H4>
<BR>
<P>You create an instance of the Data Access Object when you create an application that uses DAO and then reference that application's DAO object(s). Each time you add a reference (such as DataDict's dialog boxes, shown later), you add a reference to the object, not the object itself. With the data access object, a single CDaoDatabase object is created, and it is referenced throughout your application.
<BR>
<P>You create an instance of the DAO when you use an MFC DAO class that creates a DAO connection. Here's how you create an instance of the CDaoDatabase object data type (object class) for an existing database:
<BR>
<OL>
<LI>You declare an object variable with CDaoDatabase db.
<BR>
<BR>
<LI>You instantiate (create a pointer to) the new Database object with the CDaoDatabase::Open() member function.
<BR>
<BR>
</OL>
<BLOCKQUOTE>
<BLOCKQUOTE>
<HR ALIGN=CENTER>
<BR>
<NOTE><B>NOTE</B>
<BR>
<BR>When you declare a class variable of one of the object data types, Visual C++ initially doesn't initialize any specific database parameters to this variable.</NOTE>
<BR>
<HR ALIGN=CENTER>
</BLOCKQUOTE></BLOCKQUOTE>
<BLOCKQUOTE>
<BLOCKQUOTE>
<HR ALIGN=CENTER>
<BR>
<NOTE><B>NOTE</B>
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